1. Field of the Invention
The present invention relates to a thin film transistor (TFT), a method of fabricating the TFT, and a flat panel display having the TFT. More particularly, the present invention relates to a TFT having a dual buffer structure, a method of fabricating a TFT with a dual buffer structure, and a flat panel display having the TFT with a dual buffer structure.
2. Description of the Related Art
In general, a flat panel display may be classified as either a passive matrix type or an active matrix type based on its driving method, where the active matrix type has circuits using TFTs. TFT circuits are widely used in flat panel displays such as, for example, liquid crystal displays (LCD), organic electroluminescence displays (OELD), and so forth.
As silicon crystal fabrication technology advances, it has become possible to fabricate a polycrystalline silicon TFT among other TFTs at low temperatures similar to temperatures used to fabricate amorphous silicon TFTs. In addition, polycrystalline silicon has high electron and hole mobility as compared to amorphous silicon. Also, polycrystalline silicon may be implemented as a complementary metal-oxide semiconductor (CMOS) device, so that a TFT for driving a circuit and a TFT for driving a pixel may be formed on the same substrate at the same time.
A method for forming a polycrystalline silicon layer to be used as an active layer of a TFT typically includes depositing an amorphous silicon layer on an insulating substrate and crystallizing the amorphous silicon layer at a predetermined temperature to form a polycrystalline silicon layer therefrom. However, impurities located in the substrate before depositing the amorphous silicon layer will diffuse into adjacent semiconductor layers when the crystallization process is carried out. Such diffused impurities lead to degradation of the electrical properties of the resulting TFT. In addition, various crystallization defects occur in the polycrystalline silicon layer after the crystallization process is carried out. Furthermore, conductive layers formed in the TFT of the flat panel display cause a diffuse reflection of external light. Reflections of external light make it difficult to form a complete black in the flat panel display, and the contrast ratio of the display will be degraded due to such reflections.
Typically, at least one of the above mentioned problems is improved by preventing impurities in the substrate from diffusing into adjacent semiconductor layers. To prevent or reduce such impurity diffusion, a method of forming a buffer layer on a substrate has been typically used. Also, a passivation method has been used in order to reduce crystallization defects in the polycrystalline silicon layer. Additionally, the reflection of external light has typically been reduced and display contrast improved by forming a polarizer outside the display device, as well as by forming a black matrix inside the display device.
The formation of the buffer layer, the passivation process, the formation of the black matrix, and the arrangement of the polarizer solve the above-mentioned conventional problems, however, each of the above solutions have typically been performed by a different process. Such separation of processes causes the overall fabrication of the display device to be complicated. Additionally, as the number of process variables increases, the number of potential errors or failures in each manufacturing process also increases.